The first photos of your newborn might capture their sweet baby blues peeking out from under sleepy lids. But by their first birthday, those blue eyes might have transformed into toddler browns -- or another eye color entirely.

Eye color isn't fixed at birth. Instead, the shade can change through biological processes that involve melanin, sun exposure, and genetics, said Dr. Rebecca Chasnovitz, an associate professor of pediatrics at the University of North Carolina at Chapel Hill's School of Medicine.

"Not all babies' eyes change color," Chasnovitz said. "But if babies are born with light eyes, then sometimes their eyes will slowly darken over the first year."

Specialized cells in a baby's eyes, called melanocytes, produce a protein called melanin -- which darkens the skin, but also darkens our irises, the colored part of the eye. According to the National Library of Medicine, the more melanin, or pigment, we have in the front part of our iris, the darker our eyes will be. People with brown eyes, the most common shade in the world, have more melanin than those with lighter shades.

People with blue eyes, for example, have no melanin in the front part of their iris, according to the American Academy of Ophthalmology. People with green or hazel eyes have light brown pigment in their eyes.

As babies are exposed to sunlight, those specialized cells -- the melanocytes -- become more active, producing more melanin, Chasnovitz said. Normal sun exposure is all it takes to trigger the changes, such as regular stroller walks through the neighborhood or ambient light in the home.

Genetics add another layer to the process, determining how much melanin an individual's iris will produce. But, unlike simple inheritance patterns, eye color isn't determined by a single gene. Multiple genetic markers contribute to the final shade, making it not always easy to predict the final outcome.

In other words, if both of a baby's parents have brown eyes, that doesn't mean their offspring will also have brown eyes. "Two people with lighter eyes are more likely to have a baby with lighter eyes. Two people with darker eyes are likely to have a darker-eyed baby. But if a grandparent has light eyes, they might end up with light eyes," Chasnovitz said. "If you have a lighter eyed parent and a darker eyed parent, it's kind of a toss-up what it's going to be."

A small study in 2016 of 192 babies at Stanford University School of Medicine found that about 63 percent were born with brown eyes, about 21 percent with blue, 6 percent born with hazel or green eyes, and the remaining 10 percent could not be placed into one color category.

Parents typically start to see some changes in their child's eye color during their first six months, and the transition typically continues until the first birthday. "They'll look a little muddier if they're going to be darkening," Chasnovitz said.

There are rare exceptions where parents should take note of eye color, Chasnovitz said. For example, having one blue eye and one darker eye could indicate a genetic syndrome called Waardenburg syndrome, which requires medical attention.

Cross eyes or wandering eyes also aren't unusual in newborns, she said. By four to six months, however, if the issue hasn't resolved, she'll typically recommend going to an eye specialist.

As Chasnovitz notes, "Eyesight is the least developed of all the senses when babies are born."